Rocks typically comprise an aggregate of minerals in varying concentrations and at least some water, either absorbed or chemically bound. Early methods of analysing fragmentary rock samples have included irradiating the samples with microwave radiation for a relatively long period of time, generally in the order of several seconds or more. The microwave radiation differentially heats the rock fragments as observed by, for example, thermal imaging such as by an infra-red imaging device. Different fragments and/or areas of fragments of the rock sample are composed of different minerals and/or water content, and as such these areas will each increase in temperature to a different degree in response to the microwave irradiation.
One currently favoured application under development employs the use of microwave energy to heat conductive minerals to either directly cause particle fragmentation or to weaken the particle for subsequent fragmentation, or to induce fluid pathways to enhance later processing.
The invention described herein proposes to measure the microwave energy absorbed by each particle or rock fragment at much lower levels of microwave irradiation and use this as the basis of rejection of barren particles end possible selection between high and low grade particles containing specific minerals. The primary detection method generally includes monitoring of the degree of coupling between a suitable microwave source and the target particle. A particle which has no dielectric coupling will probably have a very low content of conductive minerals or semi-conductors such as sulphides.
Further, as minerals which behave as semi-conductors are heated, their charge carriers become more mobile and the degree of coupling to a suitable source will increase. If the rock particle which is being irradiated contains only one semi-conductor species, it should display a characteristic rate of increase of coupling. This provides an opportunity to distinguish between semi-conductor minerals as well as to estimate their presence and content.
The primary detection method advantageously provides a very rapid response. Indeed, the heating/energy absorption process is virtually instantaneous. As such, any change in energy absorption can be detected immediately. Further, there is advantageously no need to wait for heating to occur. The proposed method is also complimentary with methods for the detection of mineral content by thermal imaging. The required elements of a sorting device for carrying out the method of the invention could therefore share a particle sorting device including thermal imaging means.